The invention relates to a method for controlling transmission power in a cellular communication system in which some method for cancelling multiple access interference is utilized and in which a receiver controls the transmission power of a transmitter on the basis of a received signal.
When data transmission systems are designed and implemented, it is endeavored to maximize the number of simultaneous users on an available frequency band without compromising the quality of the transmission. An essential problem is simultaneous transmission and reception of the signals of several simultaneous users, so that the signals cause as little interference to each other as possible. Owing to this fact and the available transmission capacity, several different transmission protocols and multiple access methods have been developed, the most common of which in particular in mobile communication are the FDMA and the TDMA methods, and lately also the CDMA method. The present invention is suited for use in particular in CDMA cellular communication systems.
CDMA is a multiple access method, which is based on the spread spectrum technique, and which has been applied recently in cellular communication systems, in addition to the prior FDMA and TDMA methods. CDMA has several advantages over the prior methods, such as the simplicity of frequency planning and spectral efficiency, which results in a large capacity, i.e. the number of simultaneous users on a given frequency band.
In CDMA, the narrow-band data signal of the user is multiplied to a relatively wide band by means of a spreading code having a remarkably broader band than the data signal. Bandwidths used in known test systems are e.g. 1.25 MHz, 10 MHz and. 25 MHz. In connection with the multiplication, the data signal spreads onto the whole of the band used. All users transmit simultaneously by using the same frequency band. An individual spreading code is used on each connection between the base station and the mobile station, and the signals of the users may be distinguished from each other in the receivers on the basis of the spreading code of each user. An attempt is made for choosing the spreading codes so that they are mutually orthogonal, i.e. they do not correlate with each other.
Correlators or adapted filters in CDMA receivers implemented in a conventional way are synchronized with the desired signal, which is identified on the basis of the spreading code. The data signal is returned in the receiver onto the original band by multiplying it by the same spreading code as in the transmission phase. The signals which have been multiplied by some other spreading code neither correlate nor return to the narrow band in an ideal case. They thus appear as noise from the point of view of the desired signal. The aim is thus to detect the signal of the desired user among several interfering signals. In practice, the spreading codes are not completely non-correlated, and the signals of other users complicate the detection of the desired signal by distorting the received signal. This interference caused by the users for each other is termed as multiple access interference.
The mutual interference caused by simultaneous users for each other described above is the decisive factor for the capacity of the CDMA cellular communication system. The interference may be reduced e.g. by attempting to keep the transmission power levels of the mobile stations as low as possible by means of accurate power control. The power control may be based on some parameter measured or calculated from a received transmission, such as the received power, the signal-to-noise ratio or other quality parameter.
It is advantageous from the point of view of the capacity of the CDMA system if the base station receives the signal from all mobile stations with the same power. However, accurate and fast power control has been difficult to implement, and active reception methods based on interference cancellation thus have also been developed for reducing the interference. Such methods include, for example, interference cancellation methods (IC) and the multiuser detection (MUD). From the point of view of the present invention, the above mentioned reception methods are equal, and in the following they will be generally termed as interference cancellation methods.
In the solutions disclosed above, power control and interference cancellation are discussed as separate solutions. Conventional power control does not take into account the interference cancellation methods which are possibly used in the system, and which also improve the quality of the signal in the receiver, and conventional power control has thus resulted in an under-optimal result, in which case the available capacity has not been successfully utilized in the best possible way.